This invention relates to a power circuit having batteries for a multi-functional telephone or an accessory device connected to a telephone line and normally backed by a power source at the central telephone office.
FIG. 3 shows a conventional circuit for supplying power to a multi-functional home telephone or a telephone accessory device through a telephone line. A power source 2 at the central telephone office 1 with a steady voltage of about 48.+-.5 V is connected through coils 3 at both ends thereof and a telephone line 4 with resistance of zero to over 1000.OMEGA. to a telephone 8 and an accessory device 9 at a subscriber's home 7. The telephone 8 includes a switch 5 and has a DC resistance 6 of about 50 to 300.OMEGA.. The accessory device 9 is connected to the line 4 in parallel relationship with the telephone 8.
With a circuit structured as described above, a high voltage is applied to the accessory device 9 when the telephone 8 is in the OFF condition such as when it is disconnected or in a hold condition. As soon as the telephone 8 becomes connected, or switched on, however, the voltage on the accessory device 9 drops to less than 48 V.times. (50/1000). In other words, the accessory device 9 of FIG. 3 loses its backup voltage from the telephone office 1 as soon as a call is taken by the telephone 8.
FIG. 4 shows a previously considered power circuit with batteries 15 with which a desired voltage can be applied to such an accessory device when the telephone connected in parallel therewith is switched on and the voltage from the power source 2 at the office 1 drops substantially. With reference to FIG. 4, terminals 10 for connection with a telephone line are connected to a diode bridge 12 through a resistor 11 of 1M.OMEGA.. Numeral 14 indicates a circuit representing a telephone accessory device which requires a backup voltage and is connected to this diode bridge 12 through a first diode 13. Batteries 15 are connected to a junction between this first diode 13 and the circuit 14 through a second diode 16 which is in the opposite direction to the first diode 13. A capacitor 17 is connected in parallel with the series circuit comprised of the batteries 15 and the second diode 16. With a circuit thus constructed, the high voltage of the power source 2 from the telephone office 1 is applied to the circuit 14 of the accessor device 9 through the telephone line, the terminals 10, the diode bridge 12 and the first diode 13 when the telephone 8 is disconnected, or in the OFF condition. When the telephone 8 is switched on, or becomes connected and the voltage of the power source 2 drops, however, the batteries 15 begin to supply power to the circuit 14 of the accessory device 9 through the second diode 16, serving as a backup power source.
The disadvantage of the circuit shown in FIG. 4 is that the batteries 15 play the role of a backup power source whenever the source voltage from the central office becomes lower than the battery voltage even by a small amount. Thus, the batteries are quickly used up and they must be replaced frequently with new ones. This is both cumbersome and uneconomical.